1. Field of the Invention
The invention relates generally to pneumatic control devices and more specifically to such devices which sense a first pilot fluid and control the flow of a second, operating fluid.
2. Description of the Prior Art
Pneumatic controls and specifically pneumatic relays are well known in the art. Typical of such pneumatic relays are disclosed in U.S. Pat. No. 4,004,610, entitled "Line Control", issued Jan. 25, 1977, U.S. Pat. No. 3,877,484, entitled "Pneumatic Relay", issued Apr. 15, 1975, and U.S. Pat. No. 4,073,466, entitled "Valve", issued Feb. 14, 1978. Commonly, such devices take the form of a first piston which is acted upon by a control fluid. The piston is linked to a valve structure which controls the supply of a second fluid in response to the pressure condition of the first fluid. Typically, the first, pilot fluid can be at a relatively low pressure, for example, 25 to 30 p.s.i. whereas the second, controlled fluid can be compressed air under several hundred p.s.i. pressure. Structurally, such relays comprise piston means for activation by the pilot supply as well as axially aligned valve components such as for example, a spool valve, which controls supply and termination or venting of the second fluid to associated equipment such as a pneumatic actuator or similar control device.
In many prior art pneumatic relays, the second relatively highly pressurized fluid is controlled by an elongate stem having a plurality of circumferential grooves which receive O-rings seals. Axial translation of the stem and seals selectively closes off or connects various inlet, outlet or vent ports. In such prior art designs, a problem arises in that the repeated sliding passage of O-rings across the edges of radially oriented ports scores and abrades the O-ring seal, interfering with its sealing capability and shortening its life. The problem is especially serious in high pressure applications wherein the substantial pressure differential between one side of an O-ring seal and an exhaust port vent to atmospheric pressure will distort the O-ring seal and further aggravate damage thereto.
A second area of difficulty with prior art pneumatic relays concerns the two operational modes of such devices. In certain applications, it is necessary that once the control fluid or signal has ceased or been interrupted, even temporarily, the pneumatic relay must lock out and remain in a deactivated state until it is manually reset. Applications in control systems, especially those involved with safety devices, wherein a malfunction or shut down must be examined by an operator and a decision made regarding equipment status or startup typically require such an operational mode. The second automatic reset mode of operation of such devices comprehends conventional relay operation wherein momentary interruption or cessation of the control fluid results in a corresponding interruption or cessation of the second controlled fluid. In this mode of operation, reestablishment of the control fluid pressure or flow reestablishes the controlled fluid pressure or flow. Notwithstanding the substantial similarity of these two types of pneumatic relays, prior art relay structures exhibiting these two modes of operation are distinct from one another and require manufacturers, installers and users to provide separate relays exhibiting one or the other mode of operation. This, of course, necessitates dual inventories at all levels of utilization.
Manual reset prior art relays also display an undesirable venting function. When in the deactivated state, and subsequent to the reapplication of control air pressure, prior art relays simply pass the control air to the atmosphere until they are either reset or the control air ceases. Obviously, the quantity of control air so dumped by a plurality of deactivated valves over a period of time can be both substantial and costly.